Any pet you get will cost you money, if you care for them the right way.
When you first buy a puppy, depending on the breed, you spend quite a lot.
My parents have 2 amazing Australian Shepard's. They are brilliant dogs, but the damage they caused as puppies was outrageous.
Now add the the $1,200.00 they paid for them (perfect bloodlines), the puppy shots, the training, the food, the vet bills for booster shots and medicines to prevent heart worm every month, the neutering (a must for any responsible pet owner), the bills to replace the parts of the house they ate as puppies, and the numerous carpet cleanings until they were potty trained.
Those 2 dogs cost more than all of my fish in the first year alone. And they take a lot more work every single day than my fish do twice a week.
But I did buy my 55g tanks used, so that did cut down on the start up cost quite a bit. But it doesn't change the work involved.
Dogs, cats, fish or what ever, it all costs money to do it the right way.